How Far Back Are We Looking in Time?

When we look out into space, we’re also looking back into time. Just how far back can we see?

The Universe is a magic time window, allowing us to peer into the past. The further out we look, the further back in time we see. Despite our brains telling us things we see happen at the instant we view them, light moves at a mere 300,000 kilometers per second, which makes for a really weird time delay at great distances.

Let’s say that you’re talking with a friend who’s about a meter away. The light from your friend’s face took about 3.336 nanoseconds to reach you. You’re always seeing your loved ones 3.336 nanoseconds into the past. When you look around you, you’re not seeing the world as it is, you’re seeing the world as it was, a fraction of a second ago. And the further things are, the further back in time you’re looking.

The distance to the Moon is, on average, about 384,000 km. Light takes about 1.28 seconds to get from the Moon to the Earth. If there was a large explosion on the Moon of a secret Nazi base, you wouldn’t see it for just over a second. Even trying to communicate with someone on the Moon would be frustrating as you’d experience a delay each time you talked.

Let’s go with some larger examples. Our Sun is 8 minutes and 20 seconds away at the speed of light. You’re not seeing the Sun as it is, but how it looked more than 8 minutes ago.

On average, Mars is about 14 light minutes away from Earth. When we were watching live coverage of NASA’s Curiosity Rover landing on Mars, it wasn’t live. Curiosity landed minutes earlier, and we had to wait for the radio signals to reach us, since they travel at the speed of light.

When NASA’s New Horizons spacecraft reaches Pluto next year, it’ll be 4.6 light hours away. If we had a telescope strong enough to watch the close encounter, we’d be looking at events that happened 4.6 hours ago.

A Hubble Space Telescope image of Proxima Centauri, the closest star to Earth. Credit: ESA/Hubble & NASA
A Hubble Space Telescope image of Proxima Centauri, the closest star to Earth. Credit: ESA/Hubble & NASA

The closest star, Proxima Centauri, is more than 4.2 light-years away. This means that the Proxima Centurans don’t know who won the last US Election, or that there are going to be new Star Wars movies. They will, however, as of when this video was produced, be watching Toronto make some questionable life choices regarding its mayoral election.

The Eagle Nebula with the famous Pillars of Creation, is 7,000 light-years away. Astronomers believe that a supernova has already gone off in this region, blasting them away. Take a picture with a telescope and you’ll see them, but mostly likely they’ve been gone for thousands of years.

The core of our own Milky Way galaxy is about 25,000 light-years away. When you look at these beautiful pictures of the core of the Milky Way, you’re seeing light that may well have left before humans first settled in North America.

The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans
The Andromeda Galaxy. Credit: Adam Evans

And don’t get me started on Andromeda. That galaxy is more than 2.5 million light-years away. That light left Andromeda before we had Homo Erectus on Earth. There are galaxies out there, where aliens with powerful enough telescopes could be watching dinosaurs roaming the Earth, right now.

Here’s where it gets even more interesting. Some of the brightest objects in the sky are quasars, actively feeding supermassive black holes at the cores of galaxies. The closest is 2.5 billion light years away, but there are many much further out. Earth formed only 4.5 billion years ago, so we can see quasars shining where the light had left before the Earth even formed.

The Cosmic Microwave Background Radiation, the very edge of the observable Universe is about 13.8 billion light-years away. This light left the Universe when it was only a few hundred thousand years old, and only now has finally reached us. What’s even stranger, the place that emitted that radiation is now 46 billion light-years away from us.

So crack out your sonic screwdrivers and enjoy your time machine, Whovians. Your ability to look out into space and peer into the past. Without a finite speed of light, we wouldn’t know as much about the Universe we live in and where we came from. What moment in history do you wish you could watch? Express your answer in the form of a distance in light-years.

How Do We Know How Old Everything Is?

We hear that rocks are a certain age, and stars are another age. And the Universe itself is 13.7 billion years old. But how do astronomers figure this out?

I know it’s impolite to ask, but, how old are you? And how do you know? And doesn’t comparing your drivers license to your beautiful and informative “Year In Space” calendar feel somewhat arbitrary? How do we know old how everything is when what we observe was around long before calendars, or the Earth, or even the stars?

Scientists have pondered about the age of things since the beginning of science. When did that rock formation appear? When did that dinosaur die? How long has the Earth been around? When did the Moon form? What about the Universe? How long has that party been going on? Can I drink this beer yet, or will I go blind? How long can Spam remain edible past its expiration date?

As with distance, scientists have developed a range of tools to measure the age of stuff in the Universe. From rocks, to stars, to the Universe itself. Just like distance, it works like a ladder, where certain tools work for the youngest objects, and other tools take over for middle aged stuff, and other tools help to date the most ancient.

Let’s start with the things you can actually get your hands on, like plants, rocks, dinosaur bones and meteorites. Scientists use a technique known as radiometric dating. The nuclear age taught us how to blow up stuff real good, but it also helped understand how elements transform from one element to another through radioactive decay.

For example, there’s a version of carbon, called carbon-14. If you started with a kilo of it, after about 5,730 years, half of it would have turned into carbon-12. And then by 5,730 more years, you’d have about ¼ carbon-14 and ¾ carbon-12.

A list of the elements with their corresponding visible light emission spectra. Image Credit: MIT Wavelength Tables, NIST Atomic Spectrum Database, umop.net
A list of the elements with their corresponding visible light emission spectra. Image Credit: MIT Wavelength Tables, NIST Atomic Spectrum Database, umop.net

This is known as an element’s half-life. And so, if you measure the ratio of carbon-12 to carbon-14 in a dead tree, for example, you can calculate how long ago it lived. Different elements work for different ages. Carbon-14 works for the last 50,000 years or so, while Uranium-238 has a half-life of 4.5 billion years, and will let you date the most ancient of rocks. But what about the stuff we can’t touch, like stars?

When you use a telescope to view a star, you can break up its light into different colors, like a rainbow. This is known as a star’s spectra, and if you look carefully, you can see black lines, or gaps, which correspond to certain elements. Since they can measure the ratios of different elements, astronomers can just look at a star to see how old it is. They can measure the ratio of uranium-238 to lead-206, and know how long that star has been around. How astronomers know the age of the Universe itself is one of my favorites, and we did a whole episode on this.

Artist's conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration - D. Ducros
Artist’s conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration – D. Ducros

The short answer is, they measure the wavelength of the Cosmic Microwave Background Radiation. Since they know this used to be visible light, and has been stretched out by the expansion of the Universe, they can extrapolate back from its current wavelength to what it was at the beginning of the Universe. This tells them the age is about 13.8 billion years. Radiometric dating was a revolution for science. It finally gave us a dependable method to calculate the age of anything and everything, and finally figure out how long everything has been around.

So, fan of our videos. How old are you? Tell us in the comments below.
Thanks for watching! Never miss an episode by clicking subscribe.Our Patreon community is the reason these shows happen. We’d like to thank Ryan Finley and the rest of the members who support us in our quest to make great space and astronomy content every week. Our community members get advance access to episodes, extras, contests, and other shenanigans with Jay, myself and the rest of the team. Want to get in on the action? Click here.

How Do We Measure Distance in the Universe?

This star is X light-years away, that galaxy is X million light-years away. That beginning the Universe is X billion light-years away. But how do astronomers know?

I’m perpetually in a state where I’m talking about objects which are unimaginably far away. It’s pretty much impossible to imagine how huge some our Universe is. Our brains can comprehend the distances around us, sort of, especially when we’ve got a pile of tools to help. We can measure our height with a tape measure, or the distance along the ground using an odometer. We can get a feel for how far away 100 kilometers is because we can drive it in a pretty short period of time.

But space is really big, and for most of us, our brains can’t comprehend the full awesomeness of the cosmos, let alone measure it. So how do astronomers figure out how far away everything is? How do they know how far away planets, stars, galaxies, and even the edge of the observable Universe is? Assuming it’s all trickery? You’re bang on.

Astronomers have a bag of remarkably clever tricks and techniques to measure distance in the Universe. For them, different distances require a different methodologies. Up close, they use trigonometry, using differences in angles to puzzle out distances. They also use a variety of standard candles, those are bright objects that generate a consistent amount of light, so you can tell how far away they are. At the furthest distances, astronomers use expansion of space itself to detect distances.

Fortunately, each of these methods overlap. So you can use trigonometry to test out the closest standard candles. And you can use the most distant standard candles to verify the biggest tools. Around our Solar System, and in our neighborhood of the galaxy, astronomers use trigonometry to discover the distance to objects.

They measure the location of a star in the sky at one point of the year, and then measure again 6 months later when the Earth is on the opposite side of the Solar System. The star will have moved a tiny amount in the sky, known as parallax. Because we know the distance from one side of the Earth’s orbit to the other, we can calculate the angles, and compute the distance to the star.

I’m sure you can spot the flaw, this method falls apart when the distance is so great that the star doesn’t appear to move at all. Fortunately, astronomers shift to a different method, observing a standard candle known as a Cepheid variable. These Cepheids are special stars that dim and brighten in a known pattern. If you can measure how quickly a Cepheid pulses, you can calculate its true luminosity, and therefore its distance.

Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe.. Credit: NASA.
Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe. Credit: NASA.

Cepheids let you measure distances to nearby galaxies. Out beyond a few dozen megaparsecs, you need another tool: supernovae. In a very special type of binary star system, one star dies and becomes a white dwarf, while the other star lives on. The white dwarf begins to feed material off the partner star until it hits exactly 1.4 times the mass of the Sun. At this point, it detonates as a Type 1A supernova, generating an explosion that can be seen halfway across the Universe. Because these stars always explode with exactly the same amount of material, we can detect how far away they are, and therefore their absolute brightness.

At the greatest scales, astronomers use the Hubble Constant. This is the discovery by Edwin Hubble that the Universe is expanding in all directions. The further you look, the faster galaxies are speeding away from us. By measuring the redshift of light from a galaxy, you can tell how fast it’s moving away from us, and thus its approximate distance. At the very end of this scale is the Cosmic Microwave Background Radiation, the edge of the observable Universe, and the limit of how far we can see.

Astronomers are always looking for new types of standard candles, and have discovered all kinds of clever ways to measure distance. They measure the clustering of galaxies, beams of microwave radiation from stars, and the surface of red giant stars – all in the hopes of verifying the cosmic distance ladder. Measuring distance has been one of the toughest problems for astronomers to crack and their solutions have been absolutely ingenious. Thanks to them, we can have a sense of scale for the cosmos around us.

What concept in astronomy do you have the hardest time holding in your brain? Tell us, in the comments below.

And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

GAIA is “Go” for Science After a few Minor Hiccups

In astronomy we throw around the term “light-year” seemingly as fast as light itself travels. And yet actually measuring this distance is incredibly tricky. A star’s parallax — its tiny apparent shift once a year caused by our moving viewpoint on Earth — tells its distance more truly than any other method.

Accurate parallaxes of nearby stars form the base of the entire cosmic distance ladder out to the farthest galaxies. It’s a crucial science that’s about to take a giant leap forward. The European Space Agency’s long-awaited Gaia observatory — launched on Dec. 19, 2013 — is now ready to begin its science mission. Continue reading “GAIA is “Go” for Science After a few Minor Hiccups”

If the Moon Were Only One Pixel: a Scale Model of the Solar System

One of my favorite pet peeves is the inability of conventional models to accurately convey the gigantic scale of the Solar System. Most of us grew up with models of the planets made of wood or plastic or spray painted styrofoam balls impaled on bent wire hangers (don’t tell Mommy), or, more commonly, illustrations on posters and in textbooks. While these can be fun to look at and even show the correct relative sizes of the planets (although usually not as compared to the Sun) there’s one thing that they simply cannot relate to the viewer: space is really, really, really big.

Now there are some more human-scale models out there that do show how far the planets are from each other, but many of them require some walking, driving, or even flying to traverse their full distances. Alternatively, thanks to the magic of web pages which can be any size you like limited only by the imagination of the creator (and the patience of the viewer), accurate models can be easily presented showing the average (read: mind-blowingly enormous) distances between the planets… and no traveling or wire hangers required.

This is one of those models.* Enjoy.

Despite their similar apparent sizes in our sky, the Moon and Sun are (obviously) quite different in actual size. Which is a good thing for us. (Credit: Josh Worth)
Despite their similar apparent sizes in our sky, the Moon and Sun are (obviously) quite different in actual size. Which is a good thing for us. (Credit: Josh Worth)

Created by designer Josh Worth, “If the Moon Were Only 1 Pixel: A Tediously Accurate Scale Model of the Solar System” uses a horizontally-sliding HTML page to show how far it is from one planet to another, as well as their relative sizes, based on our Moon being just a single pixel in diameter (and everything lined up neatly in a row, which it never is.) You can use the scroll bar at the bottom of the page or arrow keys to travel the distances or, if you want to feel like you’re at least getting some exercise, scroll with your mouse or computer’s swipe pad (where applicable.) You can also use the astronomical symbols at the top of the page to “warp” to each planet.

Just try not to miss anything — it’s a surprisingly big place out there.

“You may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

– Douglas Adams

See more of Josh Worth’s work here. (HT to Alan Stern.)

*And this is another one.

Orion Revisited: Astronomers Find New Star Cluster in Front of the Orion Nebula

The well-known star-forming region of the Orion Nebula.  Credit: Canada-France-Hawaii Telescope / Coelum (J.-C. Cuillandre & G. Anselmi)

Precise distances are difficult to gauge in space, especially within the relatively local regions of the Galaxy. Stars which appear close together in the night sky may actually be separated by many hundreds or thousands of light-years, and since there’s only a limited amount of space here on Earth with which to determine distances using parallax, astronomers have to come up with other ways to figure out how far objects are, and what exactly is in front of or “behind” what.

Recently, astronomers using the 340-megapixel MegaCam on the Canada-France-Hawaii Telescope (CFHT) observed the star-forming region of the famous Orion nebula — located only about 1,500 light-years away — and determined that two massive groupings of the nebula’s stars are actually located in front of the cluster as completely separate structures… a finding that may ultimately force astronomers to rethink how the many benchmark stars located there had formed.

Although the Orion nebula is easily visible with the naked eye (as the hazy center “star” in Orion’s three-star sword, hanging perpendicular below his belt) its true nebulous nature wasn’t identified until 1610. As a vast and active star-forming region of bright dust and gas located a mere 1,500 light-years distant, the various stars within the Orion Nebula Cluster (ONC) has given astronomers invaluable benchmarks for research on many aspects of star formation.

[Read more: Astrophoto – Orion’s Bloody Massacre]

Now, CFHT observations of the Orion nebula conducted by Dr. Hervé Bouy of the European Space Astronomy Centre (ESAC) and Centre for Astrobiology (CSIC) and Dr. João Alves of the Institut für Astronomie (University of Vienna) have shown that a massive cluster of stars known as NGC 1980 is actually in front of the nebula, and is an older group of approximately 2,000 stars that is separate from the stars found within the ONC… as well as more massive than once thought.

“It is hard to see how these new observations fit into any existing theoretical model of cluster formation, and that is exciting because it suggests we might be missing something fundamental.”

– Dr. João Alves, Institut für Astronomie, University of Vienna

In addition their observations with CFHT — which were combined with previous observations with ESA’s Herschel and XMM-Newton and NASA’s Spitzer and WISE — have led to the discovery of another smaller cluster, L1641W.

According to the team’s paper, “We find that there is a rich stellar population in front of the Orion A cloud, from B-stars to M-stars, with a distinct 1) spatial distribution; 2) luminosity function; and 3) velocity dispersion from the reddened population inside the Orion A cloud. The spatial distribution of this population peaks strongly around NGC 1980 (iota Ori) and is, in all likelihood, the extended stellar content of this poorly studied cluster.”

The findings show that what has been known as Orion Nebula Cluster is actually a combination of older and newer groups of stars, possibly calling for a “revision of most of the observables in the benchmark ONC region (e.g., ages, age spread, cluster size, mass function, disk frequency, etc.)”

[Read more: Astronomers See Stars Changing Right Before Their Eyes in Orion Nebula]

“We must untangle these two mixed populations, star by star, if we are to understand the region, and star formation in clusters, and even the early stages of planet formation,” according to co-author Dr. Hervé Bouy.

The team’s article “Orion Revisited” was published in the November 2012 Astronomy & Astrophysics journal. Read the CFHT press release here.

The Canada-France-Hawaii Telescope’s Mauna Kea summit dome in September 2009. Credit: CFHT/Jean-Charles Cuillandre

Inset image: Orion nebula seen in optical – where the molecular cloud is invisible – and infrared, which shows the cloud. Any star detected in the optical in the line of sight over the region highlighted in the right panel must therefore be located in the foreground of the molecular cloud. Credit: J. Alves & H. Bouy.

How To Measure the Universe


Measuring distance doesn’t sound like a very challenging thing to do — just pick your standard unit of choice and corresponding tool calibrated to it, and see how the numbers add up. Use a meter stick, a tape measure, or perhaps take a drive, and you can get a fairly accurate answer. But in astronomy, where the distances are vast and there’s no way to take measurements in person, how do scientists know how far this is from that and what’s going where?

Luckily there are ways to figure such things out, and the methods that astronomers use are surprisingly familiar to things we experience every day.

[/caption]The video above is shared by the Royal Observatory Greenwich and shows how geometry, physics and things called “standard candles” (brilliant!) allow scientists to measure distances on cosmic scales.

Just in time for the upcoming transit of Venus, an event which also allows for some important measurements to be made of distances in our solar system, the video is part of a series of free presentations the Observatory is currently giving regarding our place in the Universe and how astronomers over the centuries have measured how oh-so-far it really is from here to there.

Video credits:
Design and direction: Richard Hogg
Animation: Robert Milne, Ross Philips, Kwok Fung Lam
Music and sound effects: George Demure
Narration and Astro-smarts: Dr. Olivia Johnson
Producer: Henry Holland

Distance to Alpha Centauri

Alpha Centauri is the closest known star system to the Solar System. Also known as Rigil Kentaurus, Alpha Centauri is actually a multiple star system. It’s certainly a binary star, with two sunlike stars orbiting one another. And there’s also a red dwarf star, Proxima Centauri, which astronomers still argue about whether it’s part of the system.

The closest star in the group is Proxima Centauri, located only 4.243 light-years from the Sun. And then the Alpha Centauri AB stars are located 4.37 light-years away.

With the unaided eye, Alpha Centauri looks like a single star. But then under the power of a telescope, it’s possible to split them and see the individual stars separately. Alpha Centauri is only really prominent in the southern skies, and below the horizon to astronomers in the north.

Alpha Centauri A is slightly larger and more luminous than the Sun, while Alpha Centauri B is smaller and cooler than the Sun. But Proxima Centauri is a tiny red dwarf star, with only 1/8th the mass of the Sun.

We’ve written several articles about the Alpha Centauri system. Here’s an article about how we might be able to detect Earthlike planets around Alpha Centauri, and here’s an article about the sounds of Alpha Centauri.

Here’s a cool image of Alpha Centauri at Astronomy Picture of the Day.

We’ve also recorded an episode of Astronomy Cast about what it might take to travel to Alpha Centauri. Listen here, Episode 145: Interstellar Travel.

Stellar Parallax

[/caption]
Parallax is the apparent difference in the position (line of sight to) an object, when the object is viewed from different locations. So, when we observe that a star has apparently moved (not to be confused with it actually having moved – proper motion), when we look at it from two different locations on the Earth’s orbit around the Sun (i.e. on different dates), that’s stellar parallax! (And if the star does not seem to have moved? Well, its parallax is zero).

The furthest apart two locations on the Earth’s orbit can be is 2 au (two astronomical units), as when observations of an object are taken six months apart. By simple trigonometry (geometry), the distance to the object being observed is just the length of the baseline divided by the tangent of the parallax angle (the angular difference in the two lines of sight) … and since parallax angles are extremely small for stars (less than one arcsecond), the tangent of the angle is the same as the angle. This gives a natural unit of distance for stars, the parsec … which is the distance at which an object has a parallax of one arcsecond when viewed from a baseline of one au.

There was a pretty hot competition, among astronomers, to be the first to measure the parallax of a star (other than the Sun), back in the 1830s; the race was won by Friedrich Bessell (remember Bessell functions?), in 1838, with a measurement of the parallax of 61 Cygni (0.314 arcsecs, in case you were wondering; two other astronomers measured the parallax of different stars in the same year).

To date, the most accurate parallaxes (~1 milli-arcsec) are the 100,000 or so obtained by the ESA’s Hipparcos mission (which operated between 1989 and 1993; results published in 1997) … Hipparcos stands for High Precision Parallax Collecting Satellite, but is also a nod to the ancient Greek astronomer Hipparchus. The follow-up mission, Gaia (target launch date: 2012) will substantially improve on this (up to a billion stars, parallaxes as small as 20 micro-arcsec). Here’s a fun fact: Gaia will measure the gravitational deflection caused the Sun … across the whole sky (and detect that due to Mars, for stars near the line sight to it)!

Universe Today has several stories on, or featuring, stellar parallax; here are a few: New Stellar Neighbors Found, Chasing an Occultation, and Happy Birthday Johannes Kepler.

Distance in Space is an Astronomy Cast episode on this very topic!

References:
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.html

Megaparsec

[/caption]
A megaparsec is a million parsecs (mega- is a prefix meaning million; think of megabyte, or megapixel), and as there are about 3.3 light-years to a parsec, a megaparsec is rather a long way. The standard abbreviation is Mpc.

Why do astronomers need to have such a large unit? When discussing distances like the size of a galaxy cluster, or a supercluster, or a void, the megaparsec is handy … just as it’s handy to use the astronomical unit (au) for solar system distances (for single galaxies, 1,000 parsecs – a kiloparsec, kpc – is a more natural scale; for cosmological distances, a gigaparsec (Gpc) is sometimes used).

Reminder: a parsec (a parallax of one arc-second, or arcsec) is a natural distance unit (for astronomers at least) because the astronomical unit (the length of the semi-major axis of the Earth’s orbit around the Sun, sorta) and arcsec are everyday units (again, for astronomers at least). Fun fact: even though the first stellar parallax distance was published in 1838, it wasn’t until 1913 that the word ‘parsec’ appeared in print!

As a parsec is approximately 3.09 x 1016 meters, a megaparsec is about 3.09 x 1022 meters.

You’ll most likely come across megaparsec first, and most often, in regard to the Hubble constant, which is the value of the slope of the straight line in a graph of the Hubble relationship (or Hubble’s Law) – redshift vs distance. As redshift is in units of kilometers per second (km/s), and as distance is in units of megaparsecs (for the sorts of distances used in the Hubble relationship), the Hubble constant is nearly always stated in units of km/s/Mpc (e.g. 72 +/- 8 km/s/Mpc, or 72 +/- 8 km s-1 Mpc-1 – that’s its estimated value from the Hubble Key Project).

John Huchra’s page on the Hubble constant is great for seeing megaparsecs in action.

Given the ubiquity of megaparsecs in extragalactic astronomy, hardly any Universe Today article on this topic is without its mention! Some examples: Chandra Confirms the Hubble Constant, Radio Astronomy Will Get a Boost With the Square Kilometer Array, and Astronomers Find New Way to Measure Cosmic Distances.

Questions Show #7, an Astronomy Cast episode, has megaparsecs in action, as does this other Questions Show.